Steering columns for motor vehicles comprise a steering shaft with a steering spindle, to whose rear end in the direction of travel, facing the driver, a steering wheel is attached for the introduction of a steering command by the driver. The steering spindle is rotatably mounted in an actuating unit, which is connected with the vehicle body via a supporting unit which for example comprises bracket parts for connection with the vehicle body, which bracket parts hold the actuating unit. Adjustment on the one hand allows setting of a comfortable, ergonomic steering wheel position relative to the driver's position in the operating position, also known as the driving or use position, in which manual steering intervention may occur. The adjustment range of possible driving or use positions is designated the “comfort position range”. On the other hand, the steering wheel may, if no manual steering intervention by the driver is needed, such as for example during autonomous driving, be moved into a stowage position outside of possible use positions of the operating position, so as to be able to put the internal vehicle space freed up thereby to an alternative use. To this end, the steering column is stowed outside the comfort position range by retracting the inner casing tube (also designated the inner tubular casing or inner casing) to the maximum extent in the longitudinal direction, i.e. forwards in the direction of travel in the direction of the longitudinal axis into the outer casing tube (also designated the outer tubular casing, outer casing unit or guide box), so maximizing the distance between the steering wheel and the driver's position. As a rule, the steering wheel of an adjustable steering column is also height-adjustable, the actuating unit being mounted on the supporting unit so as to be vertically pivotable about a pivot pin extending transversely of the longitudinal axis. By pivoting the actuating unit into the highest possible position, it may be brought into the vertical stowage position.
It is known in the prior art to provide a motorized actuating drive to adjust the actuating unit relative to the supporting unit, which actuating drive comprises an electric motor, for example, which is coupled via a gear unit to the actuating unit and enables electrical control of the steering wheel position.
In the case of a steering column of the type in question, a motorized linear actuating drive is inserted between the inner and outer casing tubes for longitudinal adjustment. The casing tubes may be moved translationally relative to one another in the direction of the longitudinal axis by the actuating drive, such that the inner casing tube may be retracted or extended telescopically relative to the outer casing tube. The actuating drive may for example take the form of a spindle drive comprising a spindle nut screwed onto a threaded spindle, which may be driven in rotation relative to the spindle by an electric motor.
One advantage of motor-adjustable steering columns is that, to increase driving and user comfort, steering wheel positions can be set in motorized manner within the comfort position range and also individual steering wheel positions can be stored electronically and be automatically retrieved and set when required. Moreover, the steering column may be retracted automatically into the stowage position outside the comfort position range.
It is in principle conceivable and possible both to adjust a motor-adjustable steering column with a sufficiently long adjustment path in the direction of the longitudinal axis in the longitudinal direction within the comfort position range and to retract the inner casing tube beyond the comfort position range maximally into the outer casing tube as far as into the stowage position. In this case, it must be ensured that, in every possible use position in the operating position, the actuating unit has the highest possible rigidity, which may be achieved by the minimum possible play between the casing tubes. At the same time, it is necessary to ensure that the actuating unit can be moved as quickly as possible into and in particular out of the stowage position, to which end the frictional force during relative displacement of the casing tubes must be as low as possible, so that the actuating drive does not have to be dimensioned to be overly powerful. Reducing the frictional force to the necessary extent for this purpose by increasing play between the casing tubes would however reduce the rigidity of the actuating unit and the natural frequency of the steering column, which is not acceptable. Due to these conflicting objectives, known motor-adjustable steering columns are not suitable for automatic stowage of a steering column.
To achieve the described conflicting objectives, WO2014/125174 A1 describes a stowable steering column which comprises separate actuating drives for adjustment in the longitudinal and vertical directions, and a third actuating drive which has an additional telescopic adjusting mechanism for stowage in the longitudinal direction. However, this known structure is complex, technically demanding and heavy.
Thus a need exists for a motor-adjustable steering column for a motor vehicle having a simpler structure at the same time as high rigidity and a high speed of adjustment into or out of the stowage position.